Struct Box

pub struct Box<T: ?Sized, A: Allocator = Global>(/* private fields */);

A pointer type that uniquely owns a heap allocation of type T.

See the module-level documentation for more.

Implementations

impl<T> Box<T>

pub fn new(x: T) -> Self

Allocates memory on the heap and then places x into it.

This doesn’t actually allocate if T is zero-sized.

Examples

let five = Box::new(5);

pub fn new_uninit() -> Box<MaybeUninit<T>>

Constructs a new box with uninitialized contents.

Examples

let mut five = Box::<u32>::new_uninit();

let five = unsafe {
    // Deferred initialization:
    five.as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5)

pub fn new_zeroed() -> Box<MaybeUninit<T>>

🔬This is a nightly-only experimental API. (new_zeroed_alloc #129396)

Constructs a new Box with uninitialized contents, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(new_zeroed_alloc)]

let zero = Box::<u32>::new_zeroed();
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0)

pub fn pin(x: T) -> Pin<Box<T>>

Constructs a new Pin<Box<T>>. If T does not implement Unpin, then x will be pinned in memory and unable to be moved.

Constructing and pinning of the Box can also be done in two steps: Box::pin(x) does the same as Box::into_pin(Box::new(x)). Consider using into_pin if you already have a Box<T>, or if you want to construct a (pinned) Box in a different way than with Box::new.

pub fn try_new(x: T) -> Result<Self, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Allocates memory on the heap then places x into it, returning an error if the allocation fails

This doesn’t actually allocate if T is zero-sized.

Examples

#![feature(allocator_api)]

let five = Box::try_new(5)?;

pub fn try_new_uninit() -> Result<Box<MaybeUninit<T>>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new box with uninitialized contents on the heap, returning an error if the allocation fails

Examples

#![feature(allocator_api)]

let mut five = Box::<u32>::try_new_uninit()?;

let five = unsafe {
    // Deferred initialization:
    five.as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5);

pub fn try_new_zeroed() -> Result<Box<MaybeUninit<T>>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new Box with uninitialized contents, with the memory being filled with 0 bytes on the heap

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api)]

let zero = Box::<u32>::try_new_zeroed()?;
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0);

impl<T, A: Allocator> Box<T, A>

pub fn new_in(x: T, alloc: A) -> Self

where A: Allocator,

🔬This is a nightly-only experimental API. (allocator_api #32838)

Allocates memory in the given allocator then places x into it.

This doesn’t actually allocate if T is zero-sized.

Examples

#![feature(allocator_api)]

use std::alloc::System;

let five = Box::new_in(5, System);

pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>

where A: Allocator,

🔬This is a nightly-only experimental API. (allocator_api #32838)

Allocates memory in the given allocator then places x into it, returning an error if the allocation fails

This doesn’t actually allocate if T is zero-sized.

Examples

#![feature(allocator_api)]

use std::alloc::System;

let five = Box::try_new_in(5, System)?;

pub fn new_uninit_in(alloc: A) -> Box<MaybeUninit<T>, A>

where A: Allocator,

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new box with uninitialized contents in the provided allocator.

Examples

#![feature(allocator_api)]

use std::alloc::System;

let mut five = Box::<u32, _>::new_uninit_in(System);

let five = unsafe {
    // Deferred initialization:
    five.as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5)

pub fn try_new_uninit_in(alloc: A) -> Result<Box<MaybeUninit<T>, A>, AllocError>

where A: Allocator,

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new box with uninitialized contents in the provided allocator, returning an error if the allocation fails

Examples

#![feature(allocator_api)]

use std::alloc::System;

let mut five = Box::<u32, _>::try_new_uninit_in(System)?;

let five = unsafe {
    // Deferred initialization:
    five.as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5);

pub fn new_zeroed_in(alloc: A) -> Box<MaybeUninit<T>, A>

where A: Allocator,

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new Box with uninitialized contents, with the memory being filled with 0 bytes in the provided allocator.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api)]

use std::alloc::System;

let zero = Box::<u32, _>::new_zeroed_in(System);
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0)

pub fn try_new_zeroed_in(alloc: A) -> Result<Box<MaybeUninit<T>, A>, AllocError>

where A: Allocator,

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new Box with uninitialized contents, with the memory being filled with 0 bytes in the provided allocator, returning an error if the allocation fails,

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api)]

use std::alloc::System;

let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0);

pub fn pin_in(x: T, alloc: A) -> Pin<Self>

where A: 'static + Allocator,

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new Pin<Box<T, A>>. If T does not implement Unpin, then x will be pinned in memory and unable to be moved.

Constructing and pinning of the Box can also be done in two steps: Box::pin_in(x, alloc) does the same as Box::into_pin(Box::new_in(x, alloc)). Consider using into_pin if you already have a Box<T, A>, or if you want to construct a (pinned) Box in a different way than with Box::new_in.

pub fn into_boxed_slice(boxed: Self) -> Box<[T], A>

🔬This is a nightly-only experimental API. (box_into_boxed_slice #71582)

Converts a Box<T> into a Box<[T]>

This conversion does not allocate on the heap and happens in place.

pub fn into_inner(boxed: Self) -> T

🔬This is a nightly-only experimental API. (box_into_inner #80437)

Consumes the Box, returning the wrapped value.

Examples

#![feature(box_into_inner)]

let c = Box::new(5);

assert_eq!(Box::into_inner(c), 5);

impl<T> Box<[T]>

pub fn new_uninit_slice(len: usize) -> Box<[MaybeUninit<T>]>

Constructs a new boxed slice with uninitialized contents.

Examples

let mut values = Box::<[u32]>::new_uninit_slice(3);

let values = unsafe {
    // Deferred initialization:
    values[0].as_mut_ptr().write(1);
    values[1].as_mut_ptr().write(2);
    values[2].as_mut_ptr().write(3);

    values.assume_init()
};

assert_eq!(*values, [1, 2, 3])

pub fn new_zeroed_slice(len: usize) -> Box<[MaybeUninit<T>]>

🔬This is a nightly-only experimental API. (new_zeroed_alloc #129396)

Constructs a new boxed slice with uninitialized contents, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(new_zeroed_alloc)]

let values = Box::<[u32]>::new_zeroed_slice(3);
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0])

pub fn try_new_uninit_slice( len: usize, ) -> Result<Box<[MaybeUninit<T>]>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new boxed slice with uninitialized contents. Returns an error if the allocation fails.

Examples

#![feature(allocator_api)]

let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
let values = unsafe {
    // Deferred initialization:
    values[0].as_mut_ptr().write(1);
    values[1].as_mut_ptr().write(2);
    values[2].as_mut_ptr().write(3);
    values.assume_init()
};

assert_eq!(*values, [1, 2, 3]);

pub fn try_new_zeroed_slice( len: usize, ) -> Result<Box<[MaybeUninit<T>]>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new boxed slice with uninitialized contents, with the memory being filled with 0 bytes. Returns an error if the allocation fails.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api)]

let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0]);

impl<T, A: Allocator> Box<[T], A>

pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[MaybeUninit<T>], A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new boxed slice with uninitialized contents in the provided allocator.

Examples

#![feature(allocator_api)]

use std::alloc::System;

let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);

let values = unsafe {
    // Deferred initialization:
    values[0].as_mut_ptr().write(1);
    values[1].as_mut_ptr().write(2);
    values[2].as_mut_ptr().write(3);

    values.assume_init()
};

assert_eq!(*values, [1, 2, 3])

pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[MaybeUninit<T>], A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new boxed slice with uninitialized contents in the provided allocator, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api)]

use std::alloc::System;

let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0])

pub fn try_new_uninit_slice_in( len: usize, alloc: A, ) -> Result<Box<[MaybeUninit<T>], A>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new boxed slice with uninitialized contents in the provided allocator. Returns an error if the allocation fails.

Examples

#![feature(allocator_api)]

use std::alloc::System;

let mut values = Box::<[u32], _>::try_new_uninit_slice_in(3, System)?;
let values = unsafe {
    // Deferred initialization:
    values[0].as_mut_ptr().write(1);
    values[1].as_mut_ptr().write(2);
    values[2].as_mut_ptr().write(3);
    values.assume_init()
};

assert_eq!(*values, [1, 2, 3]);

pub fn try_new_zeroed_slice_in( len: usize, alloc: A, ) -> Result<Box<[MaybeUninit<T>], A>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new boxed slice with uninitialized contents in the provided allocator, with the memory being filled with 0 bytes. Returns an error if the allocation fails.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api)]

use std::alloc::System;

let values = Box::<[u32], _>::try_new_zeroed_slice_in(3, System)?;
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0]);

impl<T, A: Allocator> Box<MaybeUninit<T>, A>

pub unsafe fn assume_init(self) -> Box<T, A>

Converts to Box<T, A>.

Safety

As with MaybeUninit::assume_init, it is up to the caller to guarantee that the value really is in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior.

Examples

let mut five = Box::<u32>::new_uninit();

let five: Box<u32> = unsafe {
    // Deferred initialization:
    five.as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5)

pub fn write(boxed: Self, value: T) -> Box<T, A>

🔬This is a nightly-only experimental API. (box_uninit_write #129397)

Writes the value and converts to Box<T, A>.

This method converts the box similarly to Box::assume_init but writes value into it before conversion thus guaranteeing safety. In some scenarios use of this method may improve performance because the compiler may be able to optimize copying from stack.

Examples

#![feature(box_uninit_write)]

let big_box = Box::<[usize; 1024]>::new_uninit();

let mut array = [0; 1024];
for (i, place) in array.iter_mut().enumerate() {
    *place = i;
}

// The optimizer may be able to elide this copy, so previous code writes
// to heap directly.
let big_box = Box::write(big_box, array);

for (i, x) in big_box.iter().enumerate() {
    assert_eq!(*x, i);
}

impl<T, A: Allocator> Box<[MaybeUninit<T>], A>

pub unsafe fn assume_init(self) -> Box<[T], A>

Converts to Box<[T], A>.

Safety

As with MaybeUninit::assume_init, it is up to the caller to guarantee that the values really are in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior.

Examples

let mut values = Box::<[u32]>::new_uninit_slice(3);

let values = unsafe {
    // Deferred initialization:
    values[0].as_mut_ptr().write(1);
    values[1].as_mut_ptr().write(2);
    values[2].as_mut_ptr().write(3);

    values.assume_init()
};

assert_eq!(*values, [1, 2, 3])

impl<T: ?Sized> Box<T>

pub unsafe fn from_raw(raw: *mut T) -> Self

Constructs a box from a raw pointer.

After calling this function, the raw pointer is owned by the resulting Box. Specifically, the Box destructor will call the destructor of T and free the allocated memory. For this to be safe, the memory must have been allocated in accordance with the memory layout used by Box .

Safety

This function is unsafe because improper use may lead to memory problems. For example, a double-free may occur if the function is called twice on the same raw pointer.

The safety conditions are described in the memory layout section.

Examples

Recreate a Box which was previously converted to a raw pointer using Box::into_raw:

let x = Box::new(5);
let ptr = Box::into_raw(x);
let x = unsafe { Box::from_raw(ptr) };

Manually create a Box from scratch by using the global allocator:

use std::alloc::{alloc, Layout};

unsafe {
    let ptr = alloc(Layout::new::<i32>()) as *mut i32;
    // In general .write is required to avoid attempting to destruct
    // the (uninitialized) previous contents of `ptr`, though for this
    // simple example `*ptr = 5` would have worked as well.
    ptr.write(5);
    let x = Box::from_raw(ptr);
}

impl<T: ?Sized, A: Allocator> Box<T, A>

pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self

🔬This is a nightly-only experimental API. (allocator_api #32838)

Constructs a box from a raw pointer in the given allocator.

After calling this function, the raw pointer is owned by the resulting Box. Specifically, the Box destructor will call the destructor of T and free the allocated memory. For this to be safe, the memory must have been allocated in accordance with the memory layout used by Box .

Safety

This function is unsafe because improper use may lead to memory problems. For example, a double-free may occur if the function is called twice on the same raw pointer.

Examples

Recreate a Box which was previously converted to a raw pointer using Box::into_raw_with_allocator:

#![feature(allocator_api)]

use std::alloc::System;

let x = Box::new_in(5, System);
let (ptr, alloc) = Box::into_raw_with_allocator(x);
let x = unsafe { Box::from_raw_in(ptr, alloc) };

Manually create a Box from scratch by using the system allocator:

#![feature(allocator_api, slice_ptr_get)]

use std::alloc::{Allocator, Layout, System};

unsafe {
    let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
    // In general .write is required to avoid attempting to destruct
    // the (uninitialized) previous contents of `ptr`, though for this
    // simple example `*ptr = 5` would have worked as well.
    ptr.write(5);
    let x = Box::from_raw_in(ptr, System);
}

pub fn into_raw(b: Self) -> *mut T

Consumes the Box, returning a wrapped raw pointer.

The pointer will be properly aligned and non-null.

After calling this function, the caller is responsible for the memory previously managed by the Box. In particular, the caller should properly destroy T and release the memory, taking into account the memory layout used by Box. The easiest way to do this is to convert the raw pointer back into a Box with the Box::from_raw function, allowing the Box destructor to perform the cleanup.

Note: this is an associated function, which means that you have to call it as Box::into_raw(b) instead of b.into_raw(). This is so that there is no conflict with a method on the inner type.

Examples

Converting the raw pointer back into a Box with Box::from_raw for automatic cleanup:

let x = Box::new(String::from("Hello"));
let ptr = Box::into_raw(x);
let x = unsafe { Box::from_raw(ptr) };

Manual cleanup by explicitly running the destructor and deallocating the memory:

use std::alloc::{dealloc, Layout};
use std::ptr;

let x = Box::new(String::from("Hello"));
let ptr = Box::into_raw(x);
unsafe {
    ptr::drop_in_place(ptr);
    dealloc(ptr as *mut u8, Layout::new::<String>());
}

Note: This is equivalent to the following:

let x = Box::new(String::from("Hello"));
let ptr = Box::into_raw(x);
unsafe {
    drop(Box::from_raw(ptr));
}

pub fn into_raw_with_allocator(b: Self) -> (*mut T, A)

🔬This is a nightly-only experimental API. (allocator_api #32838)

Consumes the Box, returning a wrapped raw pointer and the allocator.

The pointer will be properly aligned and non-null.

After calling this function, the caller is responsible for the memory previously managed by the Box. In particular, the caller should properly destroy T and release the memory, taking into account the memory layout used by Box. The easiest way to do this is to convert the raw pointer back into a Box with the Box::from_raw_in function, allowing the Box destructor to perform the cleanup.

Note: this is an associated function, which means that you have to call it as Box::into_raw_with_allocator(b) instead of b.into_raw_with_allocator(). This is so that there is no conflict with a method on the inner type.

Examples

Converting the raw pointer back into a Box with Box::from_raw_in for automatic cleanup:

#![feature(allocator_api)]

use std::alloc::System;

let x = Box::new_in(String::from("Hello"), System);
let (ptr, alloc) = Box::into_raw_with_allocator(x);
let x = unsafe { Box::from_raw_in(ptr, alloc) };

Manual cleanup by explicitly running the destructor and deallocating the memory:

#![feature(allocator_api)]

use std::alloc::{Allocator, Layout, System};
use std::ptr::{self, NonNull};

let x = Box::new_in(String::from("Hello"), System);
let (ptr, alloc) = Box::into_raw_with_allocator(x);
unsafe {
    ptr::drop_in_place(ptr);
    let non_null = NonNull::new_unchecked(ptr);
    alloc.deallocate(non_null.cast(), Layout::new::<String>());
}

pub fn as_mut_ptr(b: &mut Self) -> *mut T

🔬This is a nightly-only experimental API. (box_as_ptr #129090)

Returns a raw mutable pointer to the Box’s contents.

The caller must ensure that the Box outlives the pointer this function returns, or else it will end up dangling.

This method guarantees that for the purpose of the aliasing model, this method does not materialize a reference to the underlying memory, and thus the returned pointer will remain valid when mixed with other calls to as_ptr and as_mut_ptr. Note that calling other methods that materialize references to the memory may still invalidate this pointer. See the example below for how this guarantee can be used.

Examples

Due to the aliasing guarantee, the following code is legal:

#![feature(box_as_ptr)]

unsafe {
    let mut b = Box::new(0);
    let ptr1 = Box::as_mut_ptr(&mut b);
    ptr1.write(1);
    let ptr2 = Box::as_mut_ptr(&mut b);
    ptr2.write(2);
    // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
    ptr1.write(3);
}

pub fn as_ptr(b: &Self) -> *const T

🔬This is a nightly-only experimental API. (box_as_ptr #129090)

Returns a raw pointer to the Box’s contents.

The caller must ensure that the Box outlives the pointer this function returns, or else it will end up dangling.

The caller must also ensure that the memory the pointer (non-transitively) points to is never written to (except inside an UnsafeCell) using this pointer or any pointer derived from it. If you need to mutate the contents of the Box, use as_mut_ptr.

This method guarantees that for the purpose of the aliasing model, this method does not materialize a reference to the underlying memory, and thus the returned pointer will remain valid when mixed with other calls to as_ptr and as_mut_ptr. Note that calling other methods that materialize mutable references to the memory, as well as writing to this memory, may still invalidate this pointer. See the example below for how this guarantee can be used.

Examples

Due to the aliasing guarantee, the following code is legal:

#![feature(box_as_ptr)]

unsafe {
    let mut v = Box::new(0);
    let ptr1 = Box::as_ptr(&v);
    let ptr2 = Box::as_mut_ptr(&mut v);
    let _val = ptr2.read();
    // No write to this memory has happened yet, so `ptr1` is still valid.
    let _val = ptr1.read();
    // However, once we do a write...
    ptr2.write(1);
    // ... `ptr1` is no longer valid.
    // This would be UB: let _val = ptr1.read();
}

pub const fn allocator(b: &Self) -> &A

🔬This is a nightly-only experimental API. (allocator_api #32838)

Returns a reference to the underlying allocator.

Note: this is an associated function, which means that you have to call it as Box::allocator(&b) instead of b.allocator(). This is so that there is no conflict with a method on the inner type.

pub fn leak<'a>(b: Self) -> &'a mut T

where A: 'a,

Consumes and leaks the Box, returning a mutable reference, &'a mut T.

Note that the type T must outlive the chosen lifetime 'a. If the type has only static references, or none at all, then this may be chosen to be 'static.

This function is mainly useful for data that lives for the remainder of the program’s life. Dropping the returned reference will cause a memory leak. If this is not acceptable, the reference should first be wrapped with the Box::from_raw function producing a Box. This Box can then be dropped which will properly destroy T and release the allocated memory.

Note: this is an associated function, which means that you have to call it as Box::leak(b) instead of b.leak(). This is so that there is no conflict with a method on the inner type.

Examples

Simple usage:

let x = Box::new(41);
let static_ref: &'static mut usize = Box::leak(x);
*static_ref += 1;
assert_eq!(*static_ref, 42);

Unsized data:

let x = vec![1, 2, 3].into_boxed_slice();
let static_ref = Box::leak(x);
static_ref[0] = 4;
assert_eq!(*static_ref, [4, 2, 3]);

pub fn into_pin(boxed: Self) -> Pin<Self>

where A: 'static,

Converts a Box<T> into a Pin<Box<T>>. If T does not implement Unpin, then *boxed will be pinned in memory and unable to be moved.

This conversion does not allocate on the heap and happens in place.

This is also available via From.

Constructing and pinning a Box with Box::into_pin(Box::new(x)) can also be written more concisely using Box::pin(x). This into_pin method is useful if you already have a Box<T>, or you are constructing a (pinned) Box in a different way than with Box::new.

Notes

It’s not recommended that crates add an impl like From<Box<T>> for Pin<T>, as it’ll introduce an ambiguity when calling Pin::from. A demonstration of such a poor impl is shown below.

struct Foo; // A type defined in this crate.
impl From<Box<()>> for Pin<Foo> {
    fn from(_: Box<()>) -> Pin<Foo> {
        Pin::new(Foo)
    }
}

let foo = Box::new(());
let bar = Pin::from(foo);

impl<A: Allocator> Box<dyn Any, A>

pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self>

Attempts to downcast the box to a concrete type.

Examples

use std::any::Any;

fn print_if_string(value: Box<dyn Any>) {
    if let Ok(string) = value.downcast::<String>() {
        println!("String ({}): {}", string.len(), string);
    }
}

let my_string = "Hello World".to_string();
print_if_string(Box::new(my_string));
print_if_string(Box::new(0i8));

pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A>

🔬This is a nightly-only experimental API. (downcast_unchecked #90850)

Downcasts the box to a concrete type.

For a safe alternative see downcast.

Examples

#![feature(downcast_unchecked)]

use std::any::Any;

let x: Box<dyn Any> = Box::new(1_usize);

unsafe {
    assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}

Safety

The contained value must be of type T. Calling this method with the incorrect type is undefined behavior.

impl<A: Allocator> Box<dyn Any + Send, A>

pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self>

Attempts to downcast the box to a concrete type.

Examples

use std::any::Any;

fn print_if_string(value: Box<dyn Any + Send>) {
    if let Ok(string) = value.downcast::<String>() {
        println!("String ({}): {}", string.len(), string);
    }
}

let my_string = "Hello World".to_string();
print_if_string(Box::new(my_string));
print_if_string(Box::new(0i8));

pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A>

🔬This is a nightly-only experimental API. (downcast_unchecked #90850)

Downcasts the box to a concrete type.

For a safe alternative see downcast.

Examples

#![feature(downcast_unchecked)]

use std::any::Any;

let x: Box<dyn Any + Send> = Box::new(1_usize);

unsafe {
    assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}

Safety

The contained value must be of type T. Calling this method with the incorrect type is undefined behavior.

impl<A: Allocator> Box<dyn Any + Send + Sync, A>

pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self>

Attempts to downcast the box to a concrete type.

Examples

use std::any::Any;

fn print_if_string(value: Box<dyn Any + Send + Sync>) {
    if let Ok(string) = value.downcast::<String>() {
        println!("String ({}): {}", string.len(), string);
    }
}

let my_string = "Hello World".to_string();
print_if_string(Box::new(my_string));
print_if_string(Box::new(0i8));

pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A>

🔬This is a nightly-only experimental API. (downcast_unchecked #90850)

Downcasts the box to a concrete type.

For a safe alternative see downcast.

Examples

#![feature(downcast_unchecked)]

use std::any::Any;

let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);

unsafe {
    assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}

Safety

The contained value must be of type T. Calling this method with the incorrect type is undefined behavior.

Trait Implementations

impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A>

fn as_mut(&mut self) -> &mut T

Converts this type into a mutable reference of the (usually inferred) input type.

impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A>

fn as_ref(&self) -> &T

Converts this type into a shared reference of the (usually inferred) input type.

impl<Args: Tuple, F: AsyncFn<Args> + ?Sized, A: Allocator> AsyncFn<Args> for Box<F, A>

extern "rust-call" fn async_call( &self, args: Args, ) -> Self::CallRefFuture<'_>

🔬This is a nightly-only experimental API. (async_fn_traits)

Call the AsyncFn, returning a future which may borrow from the called closure.

impl<Args: Tuple, F: AsyncFnMut<Args> + ?Sized, A: Allocator> AsyncFnMut<Args> for Box<F, A>

type CallRefFuture<'a> = <F as AsyncFnMut<Args>>::CallRefFuture<'a> where Self: 'a

🔬This is a nightly-only experimental API. (async_fn_traits)

Future returned by AsyncFnMut::async_call_mut and AsyncFn::async_call.

extern "rust-call" fn async_call_mut( &mut self, args: Args, ) -> Self::CallRefFuture<'_>

🔬This is a nightly-only experimental API. (async_fn_traits)

Call the AsyncFnMut, returning a future which may borrow from the called closure.

impl<Args: Tuple, F: AsyncFnOnce<Args> + ?Sized, A: Allocator> AsyncFnOnce<Args> for Box<F, A>

type Output = <F as AsyncFnOnce<Args>>::Output

🔬This is a nightly-only experimental API. (async_fn_traits)

Output type of the called closure’s future.

type CallOnceFuture = <F as AsyncFnOnce<Args>>::CallOnceFuture

🔬This is a nightly-only experimental API. (async_fn_traits)

Future returned by AsyncFnOnce::async_call_once.

extern "rust-call" fn async_call_once( self, args: Args, ) -> Self::CallOnceFuture

🔬This is a nightly-only experimental API. (async_fn_traits)

Call the AsyncFnOnce, returning a future which may move out of the called closure.

impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator for Box<S>

type Item = <S as AsyncIterator>::Item

🔬This is a nightly-only experimental API. (async_iterator #79024)

The type of items yielded by the async iterator.

fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_>, ) -> Poll<Option<Self::Item>>

🔬This is a nightly-only experimental API. (async_iterator #79024)

Attempts to pull out the next value of this async iterator, registering the current task for wakeup if the value is not yet available, and returning None if the async iterator is exhausted.

fn size_hint(&self) -> (usize, Option<usize>)

🔬This is a nightly-only experimental API. (async_iterator #79024)

Returns the bounds on the remaining length of the async iterator.

impl<T: ?Sized, A: Allocator> Borrow<T> for Box<T, A>

fn borrow(&self) -> &T

Immutably borrows from an owned value.

impl<T: ?Sized, A: Allocator> BorrowMut<T> for Box<T, A>

fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value.

impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A>

fn clone_from(&mut self, source: &Self)

Copies source’s contents into self without creating a new allocation, so long as the two are of the same length.

Examples

let x = Box::new([5, 6, 7]);
let mut y = Box::new([8, 9, 10]);
let yp: *const [i32] = &*y;

y.clone_from(&x);

// The value is the same
assert_eq!(x, y);

// And no allocation occurred
assert_eq!(yp, &*y);

fn clone(&self) -> Self

Returns a copy of the value.

impl Clone for Box<CStr>

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A>

fn clone(&self) -> Self

Returns a new box with a clone() of this box’s contents.

Examples

let x = Box::new(5);
let y = x.clone();

// The value is the same
assert_eq!(x, y);

// But they are unique objects
assert_ne!(&*x as *const i32, &*y as *const i32);

fn clone_from(&mut self, source: &Self)

Copies source’s contents into self without creating a new allocation.

Examples

let x = Box::new(5);
let mut y = Box::new(10);
let yp: *const i32 = &*y;

y.clone_from(&x);

// The value is the same
assert_eq!(x, y);

// And no allocation occurred
assert_eq!(yp, &*y);

impl Clone for Box<str>

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<G: ?Sized + Coroutine<R> + Unpin, R, A: Allocator> Coroutine<R> for Box<G, A>

type Yield = <G as Coroutine<R>>::Yield

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

The type of value this coroutine yields.

type Return = <G as Coroutine<R>>::Return

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

The type of value this coroutine returns.

fn resume( self: Pin<&mut Self>, arg: R, ) -> CoroutineState<Self::Yield, Self::Return>

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

Resumes the execution of this coroutine.

impl<G: ?Sized + Coroutine<R>, R, A> Coroutine<R> for Pin<Box<G, A>>

where A: 'static + Allocator,

type Yield = <G as Coroutine<R>>::Yield

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

The type of value this coroutine yields.

type Return = <G as Coroutine<R>>::Return

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

The type of value this coroutine returns.

fn resume( self: Pin<&mut Self>, arg: R, ) -> CoroutineState<Self::Yield, Self::Return>

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

Resumes the execution of this coroutine.

impl<T: Debug + ?Sized, A: Allocator> Debug for Box<T, A>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T> Default for Box<[T]>

fn default() -> Self

Returns the “default value” for a type.

impl Default for Box<CStr>

fn default() -> Box<CStr>

Returns the “default value” for a type.

impl<T: Default> Default for Box<T>

fn default() -> Self

Creates a Box<T>, with the Default value for T.

impl Default for Box<str>

fn default() -> Self

Returns the “default value” for a type.

impl<T: ?Sized, A: Allocator> Deref for Box<T, A>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A>

fn deref_mut(&mut self) -> &mut T

Mutably dereferences the value.

impl<T: Display + ?Sized, A: Allocator> Display for Box<T, A>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A>

fn next_back(&mut self) -> Option<I::Item>

Removes and returns an element from the end of the iterator.

fn nth_back(&mut self, n: usize) -> Option<I::Item>

Returns the nth element from the end of the iterator.

fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>>

🔬This is a nightly-only experimental API. (iter_advance_by #77404)

Advances the iterator from the back by n elements.

fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R

where Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Output = B>,

This is the reverse version of Iterator::try_fold(): it takes elements starting from the back of the iterator.

fn rfold<B, F>(self, init: B, f: F) -> B

where Self: Sized, F: FnMut(B, Self::Item) -> B,

An iterator method that reduces the iterator’s elements to a single, final value, starting from the back.

fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>

where Self: Sized, P: FnMut(&Self::Item) -> bool,

Searches for an element of an iterator from the back that satisfies a predicate.

impl<T: ?Sized, A: Allocator> Drop for Box<T, A>

fn drop(&mut self)

Executes the destructor for this type.

impl<T: Error> Error for Box<T>

fn description(&self) -> &str

👎Deprecated since 1.42.0: use the Display impl or to_string()

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0: replaced by Error::source, which can support downcasting

fn source(&self) -> Option<&(dyn Error + 'static)>

Returns the lower-level source of this error, if any.

fn provide<'b>(&'b self, request: &mut Request<'b>)

🔬This is a nightly-only experimental API. (error_generic_member_access #99301)

Provides type-based access to context intended for error reports.

impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A>

fn len(&self) -> usize

Returns the exact remaining length of the iterator.

fn is_empty(&self) -> bool

🔬This is a nightly-only experimental API. (exact_size_is_empty #35428)

Returns true if the iterator is empty.

impl<A: Allocator> Extend<Box<str, A>> for String

fn extend<I: IntoIterator<Item = Box<str, A>>>(&mut self, iter: I)

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: A)

🔬This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A>

extern "rust-call" fn call(&self, args: Args) -> Self::Output

🔬This is a nightly-only experimental API. (fn_traits #29625)

Performs the call operation.

impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A>

extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output

🔬This is a nightly-only experimental API. (fn_traits #29625)

Performs the call operation.

impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A>

type Output = <F as FnOnce<Args>>::Output

The returned type after the call operator is used.

extern "rust-call" fn call_once(self, args: Args) -> Self::Output

🔬This is a nightly-only experimental API. (fn_traits #29625)

Performs the call operation.

impl<T: Clone> From<&[T]> for Box<[T]>

fn from(slice: &[T]) -> Box<[T]>

Converts a &[T] into a Box<[T]>

This conversion allocates on the heap and performs a copy of slice and its contents.

Examples

// create a &[u8] which will be used to create a Box<[u8]>
let slice: &[u8] = &[104, 101, 108, 108, 111];
let boxed_slice: Box<[u8]> = Box::from(slice);

println!("{boxed_slice:?}");

impl From<&CStr> for Box<CStr>

fn from(s: &CStr) -> Box<CStr>

Converts a &CStr into a Box<CStr>, by copying the contents into a newly allocated Box.

impl<'a> From<&str> for Box<dyn Error + 'a>

fn from(err: &str) -> Box<dyn Error + 'a>

Converts a str into a box of dyn Error.

Examples

use std::error::Error;
use std::mem;

let a_str_error = "a str error";
let a_boxed_error = Box::<dyn Error>::from(a_str_error);
assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))

impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a>

fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a>

Converts a str into a box of dyn Error + Send + Sync.

Examples

use std::error::Error;
use std::mem;

let a_str_error = "a str error";
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
assert!(
    mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))

impl From<&str> for Box<str>

fn from(s: &str) -> Box<str>

Converts a &str into a Box<str>

This conversion allocates on the heap and performs a copy of s.

Examples

let boxed: Box<str> = Box::from("hello");
println!("{boxed}");

impl<T, const N: usize> From<[T; N]> for Box<[T]>

fn from(array: [T; N]) -> Box<[T]>

Converts a [T; N] into a Box<[T]>

This conversion moves the array to newly heap-allocated memory.

Examples

let boxed: Box<[u8]> = Box::from([4, 2]);
println!("{boxed:?}");

impl<T, A: Allocator> From<Box<[T], A>> for Vec<T, A>

fn from(s: Box<[T], A>) -> Self

Converts a boxed slice into a vector by transferring ownership of the existing heap allocation.

Examples

let b: Box<[i32]> = vec![1, 2, 3].into_boxed_slice();
assert_eq!(Vec::from(b), vec![1, 2, 3]);

impl From<Box<CStr>> for CString

fn from(s: Box<CStr>) -> CString

Converts a Box<CStr> into a CString without copying or allocating.

impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Arc<T, A>

fn from(v: Box<T, A>) -> Arc<T, A>

Move a boxed object to a new, reference-counted allocation.

Example

let unique: Box<str> = Box::from("eggplant");
let shared: Arc<str> = Arc::from(unique);
assert_eq!("eggplant", &shared[..]);

impl<T: ?Sized, A> From<Box<T, A>> for Pin<Box<T, A>>

where A: 'static + Allocator,

fn from(boxed: Box<T, A>) -> Self

Converts a Box<T> into a Pin<Box<T>>. If T does not implement Unpin, then *boxed will be pinned in memory and unable to be moved.

This conversion does not allocate on the heap and happens in place.

This is also available via Box::into_pin.

Constructing and pinning a Box with <Pin<Box<T>>>::from(Box::new(x)) can also be written more concisely using Box::pin(x). This From implementation is useful if you already have a Box<T>, or you are constructing a (pinned) Box in a different way than with Box::new.

impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Rc<T, A>

fn from(v: Box<T, A>) -> Rc<T, A>

Move a boxed object to a new, reference counted, allocation.

Example

let original: Box<i32> = Box::new(1);
let shared: Rc<i32> = Rc::from(original);
assert_eq!(1, *shared);

impl From<Box<str>> for String

fn from(s: Box<str>) -> String

Converts the given boxed str slice to a String. It is notable that the str slice is owned.

Examples

let s1: String = String::from("hello world");
let s2: Box<str> = s1.into_boxed_str();
let s3: String = String::from(s2);

assert_eq!("hello world", s3)

impl<A: Allocator> From<Box<str, A>> for Box<[u8], A>

fn from(s: Box<str, A>) -> Self

Converts a Box<str> into a Box<[u8]>

This conversion does not allocate on the heap and happens in place.

Examples

// create a Box<str> which will be used to create a Box<[u8]>
let boxed: Box<str> = Box::from("hello");
let boxed_str: Box<[u8]> = Box::from(boxed);

// create a &[u8] which will be used to create a Box<[u8]>
let slice: &[u8] = &[104, 101, 108, 108, 111];
let boxed_slice = Box::from(slice);

assert_eq!(boxed_slice, boxed_str);

impl From<CString> for Box<CStr>

fn from(s: CString) -> Box<CStr>

Converts a CString into a Box<CStr> without copying or allocating.

impl<T: Clone> From<Cow<'_, [T]>> for Box<[T]>

fn from(cow: Cow<'_, [T]>) -> Box<[T]>

Converts a Cow<'_, [T]> into a Box<[T]>

When cow is the Cow::Borrowed variant, this conversion allocates on the heap and copies the underlying slice. Otherwise, it will try to reuse the owned Vec’s allocation.

impl From<Cow<'_, CStr>> for Box<CStr>

fn from(cow: Cow<'_, CStr>) -> Box<CStr>

Converts a Cow<'a, CStr> into a Box<CStr>, by copying the contents if they are borrowed.

impl From<Cow<'_, str>> for Box<str>

fn from(cow: Cow<'_, str>) -> Box<str>

Converts a Cow<'_, str> into a Box<str>

When cow is the Cow::Borrowed variant, this conversion allocates on the heap and copies the underlying str. Otherwise, it will try to reuse the owned String’s allocation.

Examples

use std::borrow::Cow;

let unboxed = Cow::Borrowed("hello");
let boxed: Box<str> = Box::from(unboxed);
println!("{boxed}");

let unboxed = Cow::Owned("hello".to_string());
let boxed: Box<str> = Box::from(unboxed);
println!("{boxed}");

impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + 'a>

fn from(err: Cow<'b, str>) -> Box<dyn Error + 'a>

Converts a Cow into a box of dyn Error.

Examples

use std::error::Error;
use std::mem;
use std::borrow::Cow;

let a_cow_str_error = Cow::from("a str error");
let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))

impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a>

fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a>

Converts a Cow into a box of dyn Error + Send + Sync.

Examples

use std::error::Error;
use std::mem;
use std::borrow::Cow;

let a_cow_str_error = Cow::from("a str error");
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
assert!(
    mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))

impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a>

fn from(err: E) -> Box<dyn Error + 'a>

Converts a type of Error into a box of dyn Error.

Examples

use std::error::Error;
use std::fmt;
use std::mem;

#[derive(Debug)]
struct AnError;

impl fmt::Display for AnError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "An error")
    }
}

impl Error for AnError {}

let an_error = AnError;
assert!(0 == mem::size_of_val(&an_error));
let a_boxed_error = Box::<dyn Error>::from(an_error);
assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))

impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a>

fn from(err: E) -> Box<dyn Error + Send + Sync + 'a>

Converts a type of Error + Send + Sync into a box of dyn Error + Send + Sync.

Examples

use std::error::Error;
use std::fmt;
use std::mem;

#[derive(Debug)]
struct AnError;

impl fmt::Display for AnError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "An error")
    }
}

impl Error for AnError {}

unsafe impl Send for AnError {}

unsafe impl Sync for AnError {}

let an_error = AnError;
assert!(0 == mem::size_of_val(&an_error));
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
assert!(
    mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))

impl<'a> From<String> for Box<dyn Error + 'a>

fn from(str_err: String) -> Box<dyn Error + 'a>

Converts a String into a box of dyn Error.

Examples

use std::error::Error;
use std::mem;

let a_string_error = "a string error".to_string();
let a_boxed_error = Box::<dyn Error>::from(a_string_error);
assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))

impl<'a> From<String> for Box<dyn Error + Send + Sync + 'a>

fn from(err: String) -> Box<dyn Error + Send + Sync + 'a>

Converts a String into a box of dyn Error + Send + Sync.

Examples

use std::error::Error;
use std::mem;

let a_string_error = "a string error".to_string();
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
assert!(
    mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))

impl From<String> for Box<str>

fn from(s: String) -> Box<str>

Converts the given String to a boxed str slice that is owned.

Examples

let s1: String = String::from("hello world");
let s2: Box<str> = Box::from(s1);
let s3: String = String::from(s2);

assert_eq!("hello world", s3)

impl<T> From<T> for Box<T>

fn from(t: T) -> Self

Converts a T into a Box<T>

The conversion allocates on the heap and moves t from the stack into it.

Examples

let x = 5;
let boxed = Box::new(5);

assert_eq!(Box::from(x), boxed);

impl<T, A: Allocator> From<Vec<T, A>> for Box<[T], A>

fn from(v: Vec<T, A>) -> Self

Converts a vector into a boxed slice.

Before doing the conversion, this method discards excess capacity like Vec::shrink_to_fit.

Examples

assert_eq!(Box::from(vec![1, 2, 3]), vec![1, 2, 3].into_boxed_slice());

Any excess capacity is removed:

let mut vec = Vec::with_capacity(10);
vec.extend([1, 2, 3]);

assert_eq!(Box::from(vec), vec![1, 2, 3].into_boxed_slice());

impl<'a> FromIterator<&'a char> for Box<str>

fn from_iter<T: IntoIterator<Item = &'a char>>(iter: T) -> Self

Creates a value from an iterator.

impl<'a> FromIterator<&'a str> for Box<str>

fn from_iter<T: IntoIterator<Item = &'a str>>(iter: T) -> Self

Creates a value from an iterator.

impl<A: Allocator> FromIterator<Box<str, A>> for Box<str>

fn from_iter<T: IntoIterator<Item = Box<str, A>>>(iter: T) -> Self

Creates a value from an iterator.

impl<A: Allocator> FromIterator<Box<str, A>> for String

fn from_iter<I: IntoIterator<Item = Box<str, A>>>(iter: I) -> String

Creates a value from an iterator.

impl<'a> FromIterator<Cow<'a, str>> for Box<str>

fn from_iter<T: IntoIterator<Item = Cow<'a, str>>>(iter: T) -> Self

Creates a value from an iterator.

impl<I> FromIterator<I> for Box<[I]>

fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self

Creates a value from an iterator.

impl FromIterator<String> for Box<str>

fn from_iter<T: IntoIterator<Item = String>>(iter: T) -> Self

Creates a value from an iterator.

impl FromIterator<char> for Box<str>

fn from_iter<T: IntoIterator<Item = char>>(iter: T) -> Self

Creates a value from an iterator.

impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>

type Output = <F as Future>::Output

The type of value produced on completion.

fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>

Attempts to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.

impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A>

fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A>

fn finish(&self) -> u64

Returns the hash value for the values written so far.

fn write(&mut self, bytes: &[u8])

Writes some data into this Hasher.

fn write_u8(&mut self, i: u8)

Writes a single u8 into this hasher.

fn write_u16(&mut self, i: u16)

Writes a single u16 into this hasher.

fn write_u32(&mut self, i: u32)

Writes a single u32 into this hasher.

fn write_u64(&mut self, i: u64)

Writes a single u64 into this hasher.

fn write_u128(&mut self, i: u128)

Writes a single u128 into this hasher.

fn write_usize(&mut self, i: usize)

Writes a single usize into this hasher.

fn write_i8(&mut self, i: i8)

Writes a single i8 into this hasher.

fn write_i16(&mut self, i: i16)

Writes a single i16 into this hasher.

fn write_i32(&mut self, i: i32)

Writes a single i32 into this hasher.

fn write_i64(&mut self, i: i64)

Writes a single i64 into this hasher.

fn write_i128(&mut self, i: i128)

Writes a single i128 into this hasher.

fn write_isize(&mut self, i: isize)

Writes a single isize into this hasher.

fn write_length_prefix(&mut self, len: usize)

🔬This is a nightly-only experimental API. (hasher_prefixfree_extras #96762)

Writes a length prefix into this hasher, as part of being prefix-free.

fn write_str(&mut self, s: &str)

🔬This is a nightly-only experimental API. (hasher_prefixfree_extras #96762)

Writes a single str into this hasher.

impl<'a, I, A: Allocator> IntoIterator for &'a Box<[I], A>

type IntoIter = Iter<'a, I>

Which kind of iterator are we turning this into?

type Item = &'a I

The type of the elements being iterated over.

fn into_iter(self) -> Iter<'a, I>

Creates an iterator from a value.

impl<'a, I, A: Allocator> IntoIterator for &'a mut Box<[I], A>

type IntoIter = IterMut<'a, I>

Which kind of iterator are we turning this into?

type Item = &'a mut I

The type of the elements being iterated over.

fn into_iter(self) -> IterMut<'a, I>

Creates an iterator from a value.

impl<I, A: Allocator> IntoIterator for Box<[I], A>

type IntoIter = IntoIter<I, A>

Which kind of iterator are we turning this into?

type Item = I

The type of the elements being iterated over.

fn into_iter(self) -> IntoIter<I, A>

Creates an iterator from a value.

impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A>

type Item = <I as Iterator>::Item

The type of the elements being iterated over.

fn next(&mut self) -> Option<I::Item>

Advances the iterator and returns the next value.

fn size_hint(&self) -> (usize, Option<usize>)

Returns the bounds on the remaining length of the iterator.

fn nth(&mut self, n: usize) -> Option<I::Item>

Returns the nth element of the iterator.

fn last(self) -> Option<I::Item>

Consumes the iterator, returning the last element.

fn next_chunk<const N: usize>( &mut self, ) -> Result<[Self::Item; N], IntoIter<Self::Item, N>>

where Self: Sized,

🔬This is a nightly-only experimental API. (iter_next_chunk #98326)

Advances the iterator and returns an array containing the next N values.

fn count(self) -> usize

where Self: Sized,

Consumes the iterator, counting the number of iterations and returning it.

fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>>

🔬This is a nightly-only experimental API. (iter_advance_by #77404)

Advances the iterator by n elements.

fn step_by(self, step: usize) -> StepBy<Self>

where Self: Sized,

Creates an iterator starting at the same point, but stepping by the given amount at each iteration.

fn chain<U>(self, other: U) -> Chain<Self, <U as IntoIterator>::IntoIter>

where Self: Sized, U: IntoIterator<Item = Self::Item>,

Takes two iterators and creates a new iterator over both in sequence.

fn zip<U>(self, other: U) -> Zip<Self, <U as IntoIterator>::IntoIter>

where Self: Sized, U: IntoIterator,

‘Zips up’ two iterators into a single iterator of pairs.

fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>

where Self: Sized, G: FnMut() -> Self::Item,

🔬This is a nightly-only experimental API. (iter_intersperse #79524)

Creates a new iterator which places an item generated by separator between adjacent items of the original iterator.

fn map<B, F>(self, f: F) -> Map<Self, F>

where Self: Sized, F: FnMut(Self::Item) -> B,

Takes a closure and creates an iterator which calls that closure on each element.

fn for_each<F>(self, f: F)

where Self: Sized, F: FnMut(Self::Item),

Calls a closure on each element of an iterator.

fn filter<P>(self, predicate: P) -> Filter<Self, P>

where Self: Sized, P: FnMut(&Self::Item) -> bool,

Creates an iterator which uses a closure to determine if an element should be yielded.

fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>

where Self: Sized, F: FnMut(Self::Item) -> Option<B>,

Creates an iterator that both filters and maps.

fn enumerate(self) -> Enumerate<Self>

where Self: Sized,

Creates an iterator which gives the current iteration count as well as the next value.

fn peekable(self) -> Peekable<Self>

where Self: Sized,

Creates an iterator which can use the peek and peek_mut methods to look at the next element of the iterator without consuming it. See their documentation for more information.

fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>

where Self: Sized, P: FnMut(&Self::Item) -> bool,

Creates an iterator that skips elements based on a predicate.

fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>

where Self: Sized, P: FnMut(&Self::Item) -> bool,

Creates an iterator that yields elements based on a predicate.

fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>

where Self: Sized, P: FnMut(Self::Item) -> Option<B>,

Creates an iterator that both yields elements based on a predicate and maps.

fn skip(self, n: usize) -> Skip<Self>

where Self: Sized,

Creates an iterator that skips the first n elements.

fn take(self, n: usize) -> Take<Self>

where Self: Sized,

Creates an iterator that yields the first n elements, or fewer if the underlying iterator ends sooner.

fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>

where Self: Sized, F: FnMut(&mut St, Self::Item) -> Option<B>,

An iterator adapter which, like fold, holds internal state, but unlike fold, produces a new iterator.

fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>

where Self: Sized, U: IntoIterator, F: FnMut(Self::Item) -> U,

Creates an iterator that works like map, but flattens nested structure.

fn map_windows<F, R, const N: usize>(self, f: F) -> MapWindows<Self, F, N>

where Self: Sized, F: FnMut(&[Self::Item; N]) -> R,

🔬This is a nightly-only experimental API. (iter_map_windows #87155)

Calls the given function f for each contiguous window of size N over self and returns an iterator over the outputs of f. Like slice::windows(), the windows during mapping overlap as well.

fn fuse(self) -> Fuse<Self>

where Self: Sized,

Creates an iterator which ends after the first None.

fn inspect<F>(self, f: F) -> Inspect<Self, F>

where Self: Sized, F: FnMut(&Self::Item),

Does something with each element of an iterator, passing the value on.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Borrows an iterator, rather than consuming it.

fn collect<B>(self) -> B

where B: FromIterator<Self::Item>, Self: Sized,

Transforms an iterator into a collection.

fn collect_into<E>(self, collection: &mut E) -> &mut E

where E: Extend<Self::Item>, Self: Sized,

🔬This is a nightly-only experimental API. (iter_collect_into #94780)

Collects all the items from an iterator into a collection.

fn partition<B, F>(self, f: F) -> (B, B)

where Self: Sized, B: Default + Extend<Self::Item>, F: FnMut(&Self::Item) -> bool,

Consumes an iterator, creating two collections from it.

fn is_partitioned<P>(self, predicate: P) -> bool

where Self: Sized, P: FnMut(Self::Item) -> bool,

🔬This is a nightly-only experimental API. (iter_is_partitioned #62544)

Checks if the elements of this iterator are partitioned according to the given predicate, such that all those that return true precede all those that return false.

fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R

where Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Output = B>,

An iterator method that applies a function as long as it returns successfully, producing a single, final value.

fn try_for_each<F, R>(&mut self, f: F) -> R

where Self: Sized, F: FnMut(Self::Item) -> R, R: Try<Output = ()>,

An iterator method that applies a fallible function to each item in the iterator, stopping at the first error and returning that error.

fn fold<B, F>(self, init: B, f: F) -> B

where Self: Sized, F: FnMut(B, Self::Item) -> B,

Folds every element into an accumulator by applying an operation, returning the final result.

fn reduce<F>(self, f: F) -> Option<Self::Item>

where Self: Sized, F: FnMut(Self::Item, Self::Item) -> Self::Item,

Reduces the elements to a single one, by repeatedly applying a reducing operation.

fn try_reduce<R>( &mut self, f: impl FnMut(Self::Item, Self::Item) -> R, ) -> <<R as Try>::Residual as Residual<Option<<R as Try>::Output>>>::TryType

where Self: Sized, R: Try<Output = Self::Item>, <R as Try>::Residual: Residual<Option<Self::Item>>,

🔬This is a nightly-only experimental API. (iterator_try_reduce #87053)

Reduces the elements to a single one by repeatedly applying a reducing operation. If the closure returns a failure, the failure is propagated back to the caller immediately.

fn all<F>(&mut self, f: F) -> bool

where Self: Sized, F: FnMut(Self::Item) -> bool,

Tests if every element of the iterator matches a predicate.

fn any<F>(&mut self, f: F) -> bool

where Self: Sized, F: FnMut(Self::Item) -> bool,

Tests if any element of the iterator matches a predicate.

fn find<P>(&mut self, predicate: P) -> Option<Self::Item>

where Self: Sized, P: FnMut(&Self::Item) -> bool,

Searches for an element of an iterator that satisfies a predicate.

fn find_map<B, F>(&mut self, f: F) -> Option<B>

where Self: Sized, F: FnMut(Self::Item) -> Option<B>,

Applies function to the elements of iterator and returns the first non-none result.

fn try_find<R>( &mut self, f: impl FnMut(&Self::Item) -> R, ) -> <<R as Try>::Residual as Residual<Option<Self::Item>>>::TryType

where Self: Sized, R: Try<Output = bool>, <R as Try>::Residual: Residual<Option<Self::Item>>,

🔬This is a nightly-only experimental API. (try_find #63178)

Applies function to the elements of iterator and returns the first true result or the first error.

fn position<P>(&mut self, predicate: P) -> Option<usize>

where Self: Sized, P: FnMut(Self::Item) -> bool,

Searches for an element in an iterator, returning its index.

fn max_by_key<B, F>(self, f: F) -> Option<Self::Item>

where B: Ord, Self: Sized, F: FnMut(&Self::Item) -> B,

Returns the element that gives the maximum value from the specified function.

fn max_by<F>(self, compare: F) -> Option<Self::Item>

where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering,

Returns the element that gives the maximum value with respect to the specified comparison function.

fn min_by_key<B, F>(self, f: F) -> Option<Self::Item>

where B: Ord, Self: Sized, F: FnMut(&Self::Item) -> B,

Returns the element that gives the minimum value from the specified function.

fn min_by<F>(self, compare: F) -> Option<Self::Item>

where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering,

Returns the element that gives the minimum value with respect to the specified comparison function.

fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)

where FromA: Default + Extend<A>, FromB: Default + Extend<B>, Self: Sized + Iterator<Item = (A, B)>,

Converts an iterator of pairs into a pair of containers.

fn copied<'a, T>(self) -> Copied<Self>

where T: 'a + Copy, Self: Sized + Iterator<Item = &'a T>,

Creates an iterator which copies all of its elements.

fn cloned<'a, T>(self) -> Cloned<Self>

where T: 'a + Clone, Self: Sized + Iterator<Item = &'a T>,

Creates an iterator which clones all of its elements.

fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>

where Self: Sized,

🔬This is a nightly-only experimental API. (iter_array_chunks #100450)

Returns an iterator over N elements of the iterator at a time.

fn sum<S>(self) -> S

where Self: Sized, S: Sum<Self::Item>,

Sums the elements of an iterator.

fn product<P>(self) -> P

where Self: Sized, P: Product<Self::Item>,

Iterates over the entire iterator, multiplying all the elements

fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering

where Self: Sized, I: IntoIterator, F: FnMut(Self::Item, <I as IntoIterator>::Item) -> Ordering,

🔬This is a nightly-only experimental API. (iter_order_by #64295)

Lexicographically compares the elements of this Iterator with those of another with respect to the specified comparison function.

fn partial_cmp<I>(self, other: I) -> Option<Ordering>

where I: IntoIterator, Self::Item: PartialOrd<<I as IntoIterator>::Item>, Self: Sized,

Lexicographically compares the PartialOrd elements of this Iterator with those of another. The comparison works like short-circuit evaluation, returning a result without comparing the remaining elements. As soon as an order can be determined, the evaluation stops and a result is returned.

fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>

where Self: Sized, I: IntoIterator, F: FnMut(Self::Item, <I as IntoIterator>::Item) -> Option<Ordering>,

🔬This is a nightly-only experimental API. (iter_order_by #64295)

Lexicographically compares the elements of this Iterator with those of another with respect to the specified comparison function.

fn eq<I>(self, other: I) -> bool

where I: IntoIterator, Self::Item: PartialEq<<I as IntoIterator>::Item>, Self: Sized,

Determines if the elements of this Iterator are equal to those of another.

fn eq_by<I, F>(self, other: I, eq: F) -> bool

where Self: Sized, I: IntoIterator, F: FnMut(Self::Item, <I as IntoIterator>::Item) -> bool,

🔬This is a nightly-only experimental API. (iter_order_by #64295)

Determines if the elements of this Iterator are equal to those of another with respect to the specified equality function.

fn ne<I>(self, other: I) -> bool

where I: IntoIterator, Self::Item: PartialEq<<I as IntoIterator>::Item>, Self: Sized,

Determines if the elements of this Iterator are not equal to those of another.

fn lt<I>(self, other: I) -> bool

where I: IntoIterator, Self::Item: PartialOrd<<I as IntoIterator>::Item>, Self: Sized,

Determines if the elements of this Iterator are lexicographically less than those of another.

fn le<I>(self, other: I) -> bool

where I: IntoIterator, Self::Item: PartialOrd<<I as IntoIterator>::Item>, Self: Sized,

Determines if the elements of this Iterator are lexicographically less or equal to those of another.

fn gt<I>(self, other: I) -> bool

where I: IntoIterator, Self::Item: PartialOrd<<I as IntoIterator>::Item>, Self: Sized,

Determines if the elements of this Iterator are lexicographically greater than those of another.

fn ge<I>(self, other: I) -> bool

where I: IntoIterator, Self::Item: PartialOrd<<I as IntoIterator>::Item>, Self: Sized,

Determines if the elements of this Iterator are lexicographically greater than or equal to those of another.

fn is_sorted_by<F>(self, compare: F) -> bool

where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> bool,

Checks if the elements of this iterator are sorted using the given comparator function.

fn is_sorted_by_key<F, K>(self, f: F) -> bool

where Self: Sized, F: FnMut(Self::Item) -> K, K: PartialOrd,

Checks if the elements of this iterator are sorted using the given key extraction function.

impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A>

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized + PartialOrd,

Restrict a value to a certain interval.

impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A>

fn eq(&self, other: &Self) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Self) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A>

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Self) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Self) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn ge(&self, other: &Self) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

fn gt(&self, other: &Self) -> bool

Tests greater than (for self and other) and is used by the > operator.

impl<T: ?Sized, A: Allocator> Pointer for Box<T, A>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]>

fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error>

Attempts to convert a Box<[T]> into a Box<[T; N]>.

The conversion occurs in-place and does not require a new memory allocation.

Errors

Returns the old Box<[T]> in the Err variant if boxed_slice.len() does not equal N.

type Error = Box<[T]>

The type returned in the event of a conversion error.

impl<T, const N: usize> TryFrom<Vec<T>> for Box<[T; N]>

fn try_from(vec: Vec<T>) -> Result<Self, Self::Error>

Attempts to convert a Vec<T> into a Box<[T; N]>.

Like Vec::into_boxed_slice, this is in-place if vec.capacity() == N, but will require a reallocation otherwise.

Errors

Returns the original Vec<T> in the Err variant if boxed_slice.len() does not equal N.

Examples

This can be used with vec! to create an array on the heap:

let state: Box<[f32; 100]> = vec![1.0; 100].try_into().unwrap();
assert_eq!(state.len(), 100);

type Error = Vec<T>

The type returned in the event of a conversion error.

impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A>

impl<T: ?Sized, A: Allocator> DerefPure for Box<T, A>

impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global>

impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A>

impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A>

impl<'a, I, A: Allocator> !Iterator for &'a Box<[I], A>

This implementation is required to make sure that the &Box<[I]>: IntoIterator implementation doesn’t overlap with IntoIterator for T where T: Iterator blanket.

impl<'a, I, A: Allocator> !Iterator for &'a mut Box<[I], A>

This implementation is required to make sure that the &mut Box<[I]>: IntoIterator implementation doesn’t overlap with IntoIterator for T where T: Iterator blanket.

impl<I, A: Allocator> !Iterator for Box<[I], A>

This implementation is required to make sure that the Box<[I]>: IntoIterator implementation doesn’t overlap with IntoIterator for T where T: Iterator blanket.

impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Box<T, A>

impl<T: ?Sized, A: Allocator> Unpin for Box<T, A>